I 
Tel: 01988 500594
Galloway Astronomy Centre SCOPE SHOP
Tel: 01988 500594
© Galloway Astronomy Centre 2016          All images are copyright – M Alexander unless otherwise stated
Improving Your “Cheapy” Telescope  Low cost telescopes of say £50 to £120 don’t tend to be very good quality. These are the type you can buy in department stores, some camera shops, ebay and even supermarkets.  If you have one of these there are a few things you can do straight away to improve it. All the parts as indicated in Image #1 that make up a telescope are important not just the tube you look  through.  A brief extra note - in the Image #1 you will see a ring with a hole in it at the front of the telescope. If after  you remove the front cover you still see this on yours stick your finger in the hole and hook out the rest of the cover. It should pull away easily. The front of your reflector should look like image #2. The whole of the cover  should be removed every time you use the telescope. (I only mention this as I see it all the time) Mounts Cheap telescopes often have Altitude - Azimuth mounts (AZ mount) which can be a bit flimsy and wobbly. See  Image #1. These points will improve an equatorial mount (the type with a counter weight) too. The wobble is mostly due to the tripod legs not being held rigidly. Avoid extending the tripod legs too high as  this makes the wobble worst. The thin metal leg braces and accessories tray which should help stabilise the  mount often have little effect. Also avoid using the telescope on a hard surface like concrete, on grass push  the spikes on the end of the legs into the ground. Try each of the following one at a time as just one might be enough: - Replace the tray with a larger triangular piece of 2 mm thick plywood (shaped as in Image #3) this will hold  the legs rigid. The flat corners need to be the width of each tripod leg. - Tighten all wingnuts and screws on the tripod firmly without over tightening to minimize flex in the whole  structure. - To minimize vibrations hang a large plastic container with a handle like the ones for milk or fabric conditioner  and half fill it with sand. Now hang it from the centre of the mount. By the way, you should not really use the tray to hold your eyepieces - it allows them to get cold and damp. Better to keep them in your pocket with the covers on Finder Your telescope is almost certainly has a tiny finderscope. As you can see from the picture (image #4) it looks  like a mini telescope mounted on top of the telescope itself. There is are hidden problems with this type of  finderscope. 1) The small lens visible at the front of the finderscope does not let in very much of the light you need to be  able to locate the objects you want to see. 2) Worst still if you look closely just behind the lens is a disc with a small hole in the centre. This further  restricting light entering your eye. It is there to stop bright objects like the Moon and planets from having a  rainbow of colours around them as the lens is of poor quality. However, the colours are not really much of a  problem, better to take it our and let in more light.  Removing the baffle is very easy to do following these steps: - First unscrew the end you view through from the tube - Looking inside the now open end of the tube you will either see a small black disc close to the lens or a short  black tube (see image #5)   - Now unscrew the front fitting that hold the lens in place. Do this with the tube vertical and the lens end at  the bottom. This is so the lens does not fall out. Don’t worry if it does, looking at it you will see the lens is flat  on one side and curved on the other. Drop the lens back in with the curved side down. - Now using a pencil or metal rod that is long enough you can knock out the disc (or tube) baffle. - Reattach the front lens end then the viewing end. - Looking through the finderscope it should be noticeable how much bright is the view. You may not be aware that the finderscope can be focused. From Image #6 you will see that the end you look  through rotates in both directions separately to the part that you detached to get inside to find the baffle. To  focus the finderscope point it toward a distant object and rotate the lens end until the image is sharp. There is another option - replace the finderscope with a red dot finder with a 2 hole base (Image #7). The same type of finderscope is fitted to many Skywatcher telescopes. It works by putting a red dot onto a  clear screen which you look through, you put the dot on the object you want to observe. As with the  finderscope it needs to be aligned with the telescope. Focuser The focuser is the part into which you put the eyepieces to view through the telescope (see Image #8). There is a small knob on either side which rotates to move the eyepiece in and out. This can feel stiff which will  cause you to shake the telescope making accurate focusing difficult. Between the two knob on the underside  will be two (or four) cross headed screws. Unscrew all the screws by about a half turn, that should give a  smoother feel to the focus. If you unscrew them too far you will see the focuser knobs are able to wobble up  and down. Just tighten the screws a little until the wobble stops. Eyepieces Eyepieces (please don’t call them lenses) are extremely important as they provide the magnification you need  to view the object. There are several common problems with the eyepieces supplied with cheap telescopes: 1) Some eyepieces are non standard 0.965” diameter. They should be 1.25” diameter. See Image #9  2) they have often tiny lenses so you have to get your eye very close to the lens look through it. This is a  particular problem if you wear glasses.  3) the magnifications they achieve do not always give the best views. If the instruction sheet does not tell you the diameter of the eyepieces you can easily check by holding a ruler  across the chrome tube of the eyepiece. If it is less than 1.25” then you have 0.965” eyepieces. These need to be replaced as soon as you can. Obviously a 1.25” eyepiece will not fit straight into a 0.965” focuser - an adapter (see image #10) is required  allowing the telescope to take the larger eyepieces. See the bottom of this page for alternative eyepieces. Magnification (M) can be calculated by     M= focal length of the telescope            focal length of the eyepiece            For example a telescope of 700mm focal length (FL) and an eyepiece of 4mm gives you an M = 175  (ie  700/4). This is written as 175x - meaning 175 times magnification  For a telescope of 70 or 76mm diameter this is much too high. The image will be faint and probably will not  focus sharply. The good guide to the maximum magnification of any telescope can be found by dividing the aperture  (telescope front lens or mirror diameter) by 25 then multiply by 30  eg 76mm reflector telescope  76/25 = 3  3 x 30 = 90  If your maximum magnification does not exceed 90 your telescope will give you enjoyable bright, sharp views.  Use the first calculation to see what magnification each of your eyepieces will give. Your kit may also include a 2x (or 3x) Barlow lens. This is a special lens which when used with an eyepiece doubles (2x) or triples (3x) the  normal magnification of that eyepiece.  Here is an example of the above:  You have a 76/700 reflecting telescope - this means the mirror is 76mm in diameter with a focal length of  700mm  There are 3 eyepieces with labels reading 20mm, 12.5mm & 4mm - this means focal lengths of these  eyepieces are 20mm 12.5mm & 4mm  There is also a 2 x Barlow - this will triple the magnification of any of the eyepieces Eyepiece FL Magnification 20mm  35x  (=700/20)  20mm + 2x Barlow  70x (=35x2)  12.5mm  56x  (=700/20)  12.5mm + 2x Barlow  112x (=35x2)  4mm  175x  (=700/20)  4mm + 2x Barlow  350x (=35x2)  You may think a magnification of 35x will not show you much, but it depends on the size of the object. The  Moon will show a lot of detail and allow you to decide where you what to look more closely. If you have not too much light pollution large star clusters will fill the view.  As you will see the 4mm eyepiece on its own far exceeds the 90x maximum for this size of telescope. Although  the use of the Barlow lens with the 12.5mm eyepiece exceeds this, on bright objects like the Moon you may  find the view is OK.  A quick calculation will show that a 20mm eyepiece with a 3x Barlow lens gives 105x magnification, which  means you could not use it with any of the other eyepieces. Better Eyepieces A quick search of the internet for Eyepieces will soon show you a confusing number of types and focal lengths, so which one is best for you? First off don’t spend a huge amount on an eyepiece. An eyepiece costing as  much as your telescope will not improve the view as much as you might hope. It is more sensible to save that  money for a better telescope in the future. Nor do you need a box full of eyepieces - two or at most three  covering low, medium and high magnification is all you need. If you are happy with the 20mm eyepiece that  came with the telescope why replace it? It is the mid and high magnification eyepieces and probably the  Barlow lens too that usually need replacing.  So what other sizes do you need? You can work it out by reversing the magnification equation. Eyepiece FL = Focal length of the telescope           Magnification Eg You want 90x magnification 700/90 = 7.7mm  Eyepieces FL are usually whole numbers so you cannot buy a 7.7 mm eyepiece only a 7mm or 8mm. Rather  than buy one that size you can achieve the same magnification using the Barlow lens and you get the  advantage of the lower magnification using it on its own too. Therefore, I would suggest a 15mm eyepiece.  This gives 47x on its own and 94x with the Barlow lens.  If the 20mm eyepiece is just adequate, but you would like to change it then buy a 25mm eyepiece as it will  give you lower magnification and a larger field of view to locate an object. A 25mm eyepiece gives 28x on its  own and 56x with the Barlow lens. This later is the same as the 12.5mm eyepiece that came with the  telescope so you have effectively replaced that too. So with just two new eyepieces and possibly a new Barlow lens you have a wide range of magnification from  low to the maximum the telescope can easily handle.  The industry standard for eyepieces is a barrel diameter of 1.25” so they can be used in any telescope you buy in the future. How much do eyepieces cost? We have a special offer for you:   25mm Eyepiece  £18  15mm Eyepiece  £18  Basic 2x Barlow Lens  £10  Higher Quality 2x Barlow Lens  £25  Red Dot Finder (2 hole type)  £15  0.965” to 1.25” Eyepiece Adaptor £12 Delivery charges apply 
If you need any help or advice on modifications, eyepieces or buying a new telescope please contact me. 
Image #1
Image #2
Image #3
Image #4
Image #5
Image #6
I mage #7
Slacken these screws
Image #9
Image #10
Scopes Introduction Basics Finally Scope Mods Skywatcher Celestron Optical Hardware Scope Mods
Galloway Astronomy Centre SCOPE SHOP
Tel: 01988 500594
Improving Your “Cheapy” Telescope  Low cost telescopes of say £50 to £120 don’t tend to be very good  quality. These are the type you can buy in department stores, some  camera shops, ebay and even supermarkets.  If you have one of these there are a few things you can do straight  away to improve it. All the parts as indicated in Image #1 that make up a telescope are  important not just the tube you look through. A brief extra note - in the Image #1 you will see a ring with a hole in it  at the front of the telescope. If after you remove the front cover you  still see this on yours stick your finger in the hole and hook out the rest of the cover. It should pull away easily. The front of your reflector  should look like image #2. The whole of the cover should be removed  every time you use the telescope. (I only mention this as I see it all the time)  Mounts Cheap telescopes often have Altitude - Azimuth mounts (AZ mount)  which can be a bit flimsy and wobbly. See Image #1. These points will  improve an equatorial mount (the type with a counter weight) too. The wobble is mostly due to the tripod legs not being held rigidly. Avoid  extending the tripod legs too high as this makes the wobble worst. The  thin metal leg braces and accessories tray which should help stabilise  the mount often have little effect. Also avoid using the telescope on a  hard surface like concrete, on grass push the spikes on the end of the  legs into the ground. Try each of the following one at a time as just one might be enough: - Replace the tray with a larger triangular piece of 2 mm thick plywood  (shaped as in Image #3) this will hold the  legs rigid. The flat corners need to be the width of each tripod leg. - Tighten all wingnuts and screws on the tripod firmly without over  tightening to minimize flex in the whole structure.  - To minimize vibrations hang a large plastic container with a handle  like the ones for milk or fabric conditioner and half fill it with sand. Now  hang it from the centre of the mount. By the way, you should not really use the tray to hold your eyepieces -  it allows them to get cold and damp. Better to keep them in your pocket with the covers on Finder Your telescope is almost certainly has a tiny finderscope. As you can  see from the picture (image #4) it looks like a mini telescope mounted  on top of the telescope itself. There is are hidden problems with this  type of finderscope. 1) The problem is the small lens visible at the front of the finderscope.  Being small it does not let in very much light which you need to be able  to locate the objects you want to see. 2) Worst still if you look closely just behind the lens with be a disc  which is further restricting light entering your eye. It is there to stop  bright objects like the Moon and planets from having a rainbow of  colours around them as the lend is of poor quality. However, the colours are not really much of a problem, better to let all the light in.  Removing the baffle is very easy to do following these steps: - First unscrew the end you view through from the tube - Looking inside the now open end of the tube you will either see a  small black disc close to the lens or a short black tube (see image #5) - Now unscrew the front fitting that hold the lens in place. Do this with  the tube vertical and the lens end at the bottom. This is so the lens  does not fall out. Don’t worry if it does, looking at it you will see the  lens is flat on one side and curved on the other. Drop the lens back in  with the curved side down. - Now using a pencil or metal rod that is long enough you can knock  out the disc (or tube) baffle. - Reattach the front lens end then the viewing end. - Looking through the finderscope it should be noticeable how much  bright is the view. You may not be aware that the finderscope can be focused. From  Image #6 you will see that the end you look through rotates in both  directions separately to the part that you detached to get inside to find the baffle. To focus the finderscope point it toward a distant object and rotate the lens end until the image is sharp. Focuser The focuser is the part into which you put the eyepieces to view  through the telescope (see Image #1). There is a small knob on either  side which rotates to move the eyepiece in and out. This can feel stiff  which will cause you to shake the telescope making accurate focusing  difficult. Between the two knob on the underside will be two (or four)  cross headed screws. Unscrew all the screws by about a half turn, that  should give a smoother feel to the focus. If you unscrew them too far  you will see the focuser knobs are able to wobble up and down. Just  tighten the screws a little until the wobble stops. Eyepieces Eyepieces (please don’t call them lenses) are extremely important as  they provide the magnification you need to view the object. There are  several common problems with the eyepieces supplied with cheap  telescopes: 1) Some eyepieces are non standard 0.965” diameter. They should be  1.25” diameter. See Image #7  2) they have often tiny lenses so you have to get your eye very close to the lens look through it. This is a particular problem if you wear  glasses. 3) the magnifications they achieve do not always give the best views. If the instruction sheet does not tell you the diameter of the eyepieces  you can easily check by holding a ruler across the chrome tube of the  eyepiece. If it is less than 1.25” then you have 0.965” eyepieces. These need to be replaced as soon as you can. See the bottom of this page  for alternative eyepieces. Magnification (M) can be calculated by     M= focal length of the telescope                   focal length of the eyepiece                            For example a telescope of 700mm focal length (FL) and an eyepiece of  4mm gives you an M = 175  (ie 700/4). This is written as 175x -  meaning 175 times magnification  For a telescope of 70 or 76mm diameter this is much too high. The  image will be faint and probably will not focus sharply.  The good guide to the maximum magnification of any telescope can be  found by dividing the aperture (telescope front lens or mirror diameter)  by 25 then multiply by 30  eg 76mm reflector telescope  76/25 = 3  3 x 30 = 90  If your maximum magnification does not exceed 90 your telescope will  give you enjoyable bright, sharp views.  Use the first calculation to see what magnification each of your  eyepieces will give. Your kit may also include a 2x (or 3x) Barlow lens.  This is a special lens which when used with an eyepiece doubles (2x) or  triples (3x) the normal magnification of that eyepiece.  Here is an example of the above:  You have a 76/700 reflecting telescope - this means the mirror is  76mm in diameter with a focal length of 700mm  There are 3 eyepieces with labels reading 20mm, 12.5mm & 4mm -  this means focal lengths of these eyepieces are 20mm 12.5mm & 4mm  There is also a 2 x Barlow - this will triple the magnification of any of  the eyepieces Eyepiece FL Magnification 20mm  35x  (=700/20)  20mm + 2x Barlow  70x (=35x2)  12.5mm  56x  (=700/20)  12.5mm + 2x Barlow  112x (=35x2)  4mm  175x  (=700/20)  4mm + 2x Barlow  350x (=35x2)  You may think a magnification of 35x will not show you much, but it  depends on the size of the object. The Moon will show a lot of detail  and allow you to decide where you what to look more closely. If you  have not too much light pollution large star clusters will fill the view. As you will see the 4mm eyepiece on its own far exceeds the 90x  maximum for this size of telescope. Although the use of the Barlow lens  with the 12.5mm eyepiece exceeds this, on bright objects like the Moon you may find the view is OK.  A quick calculation will show that a 20mm eyepiece with a 3x Barlow  lens gives 105x magnification, which means you could not use it with  any of the other eyepieces. Better Eyepieces A quick search of the internet for Eyepieces will soon show you a  confusing number of types and focal lengths, so which one is best for  you? First off don’t spend a huge amount on an eyepiece. An eyepiece  costing as much as your telescope will not improve the view as much  as you might hope. It is more sensible to save that money for a better  telescope in the future. Nor do you need a box full of eyepieces - two or at most three covering low, medium and high magnification is all you  need. If you are happy with the 20mm eyepiece that came with the  telescope why replace it? It is the mid and high magnification eyepieces and probably the Barlow lens too that usually need replacing.  So what other sizes do you need? You can work it out by reversing the  magnification equation.  Eyepiece FL = Focal length of the telescope           Magnification Eg You want 90x magnification 700/90 = 7.7mm  Eyepieces FL are usually whole numbers so you cannot buy a 7.7 mm  eyepiece only a 7mm or 8mm. Rather than buy one that size you can  achieve the same magnification using the Barlow lens and you get the  advantage of the lower magnification using it on its own too. Therefore, I would suggest a 15mm eyepiece. This gives 47x on its own and 94x  with the Barlow lens.  If the 20mm eyepiece is just adequate, but you would like to change it  then buy a 25mm eyepiece as it will give you lower magnification and a  larger field of view to locate an object. A 25mm eyepiece gives 28x on  its own and 56x with the Barlow lens. This later is the same as the  12.5mm eyepiece that came with the telescope so you have effectively  replaced that too. So with just two new eyepieces and possibly a new Barlow lens you  have a wide range of magnification from low to the maximum the  telescope can easily handle. The industry standard for eyepieces is a barrel diameter of 1.25” so  they can be used in any telescope you buy in the future. How much do eyepieces cost? We have a special offer :    25mm Eyepiece  £18  15mm Eyepiece  £18  Basic 2x Barlow Lens  £10  Higher Quality Barlow Lens  £25  Delivery charges apply  If you need any help or advice on modifications, eyepieces or buying a  new telescope please contact me.
Scopes Introduction Basics Finally Scope Mods Skywatcher Celestron Optical Hardware Scope Mods
© Galloway Astronomy Centre 2016 All images are copyright – M Alexander unless otherwise stated
Galloway Astronomy Centre SCOPE SHOP
Tel: 01988 500594
Improving Your “Cheapy” Telescope  Low cost telescopes of say £50 to £120 don’t tend to be very  good quality. These are the type you can buy in department  stores, some camera shops, ebay and even supermarkets.  If you have one of these there are a few things you can do  straight away to improve it. All the parts as indicated in Image #1 that make up a telescope  are important not just the tube you look through. A brief extra note - in the Image #1 you  will see a ring with a hole in it at the  front of the telescope. If after you  remove the front cover you still see this  on yours stick your finger in the hole and  hook out the rest of the cover. It should  pull away easily. The front of your  reflector should look like image #2. The  whole of the cover should be removed  every time you use the telescope. (I only  mention this as I see it all the time)  Mounts Cheap telescopes often have Altitude -  Azimuth mounts (AZ mount) which can be a  bit flimsy and wobbly. See Image #1. These  points will improve an equatorial mount (the  type with a counter weight) too. The wobble is mostly due to the tripod legs  not being held rigidly. Avoid extending the  tripod legs too high as this makes the wobble  worst. The thin metal leg braces and  accessories tray which should help stabilise  the mount often have little effect. Also avoid  using the telescope on a hard surface like  concrete, on grass push the spikes on the end  of the legs into the ground. Try each of the following one at a time as just  one might be enough:  - Replace the tray with a larger triangular piece of 2 mm thick  plywood (shaped as in Image #3) this will hold the  legs rigid. The flat corners need to be the width of each tripod  leg. - Tighten all wingnuts and screws on the tripod firmly without  over tightening to minimize flex in the whole structure.  - To minimize vibrations hang a large plastic container with a  handle like the ones for milk or fabric conditioner and half fill it  with sand. Now hang it from the centre of the mount. By the way, you should not really use the tray to hold your  eyepieces - it allows them to get cold and damp. Better to keep them in your pocket with the covers on Finder Your telescope is almost certainly has a tiny finderscope. As you  can see from the picture (image #4) it looks like a mini telescope  mounted on top of the telescope itself. There is are hidden  problems with this type of finderscope. 1) The small lens visible at the front of the  finderscope does not let in very much of the  light you need to be able to locate the objects you want to see. 2) Worst still if you look closely just behind  the lens is a disc with a small hole in the  centre. This further restricting light entering  your eye. It is there to stop bright objects like the Moon and planets from having a rainbow of  colours around them as the lens is of poor quality. However, the  colours are not really much of a problem, better to take it our and let in more light.  Removing the baffle is very easy to do following these steps: - First unscrew the end you view through from the tube - Looking inside the now open end of the tube  you will either see a small black disc close to  the lens or a short black tube (see image #5) - Now unscrew the front fitting that hold the  lens in place. Do this with the tube vertical  and the lens end at the bottom. This is so the  lens does not fall out. Don’t worry if it does,  looking at it you will see the lens is flat on one side and curved on the other. Drop the lens  back in with the curved side down. - Now using a pencil or metal rod that is long enough you can  knock out the disc (or tube) baffle. - Reattach the front lens end then the viewing end. - Looking through the finderscope it should be  noticeable how much bright is the view. You may not be aware that the finderscope  can be focused. From Image #6 you will see  that the end you look through rotates in both  directions separately to the part that you  detached to get inside to find the baffle. To  focus the finderscope point it toward a distant object and rotate  the lens end until the image is sharp.  There is another option - replace the  finderscope with a red dot finder with a 2 hole  base (Image #7).  The same type of finderscope is fitted to many Skywatcher telescopes. It works by putting a  red dot onto a clear screen which you look  through, you put the dot on the object you  want to observe. As with the finderscope it  needs to be aligned with the telescope. Focuser The focuser is the part into which you put the  eyepieces to view through the telescope (see  Image #8). There is a small knob on either side  which rotates to move the eyepiece in and out.  This can feel stiff which will cause you to shake  the telescope making accurate focusing difficult.  Between the two knob on the underside will be  two (or four) cross headed screws. Unscrew all  the screws by about a half turn, that should give a smoother feel to the focus. If you unscrew  them too far you will see the focuser knobs are  able to wobble up and down. Just tighten the  screws a little until the wobble stops. Eyepieces Eyepieces (please don’t call them lenses) are  extremely important as they provide the  magnification you need to view the object.  There are several common problems with the  eyepieces supplied with cheap telescopes: 1) Some eyepieces are non standard 0.965”  diameter. They should be 1.25” diameter. See  Image #9  2) they have often tiny lenses so you have to  get your eye very close to the lens look  through it. This is a particular problem if you  wear glasses. 3) the magnifications they achieve do not  always give the best views. If the instruction sheet does not tell you the  diameter of the eyepieces you can easily check by  holding a ruler across the chrome tube of the  eyepiece. If it is less than 1.25” then you have  0.965” eyepieces. These need to be replaced as  soon as you can. Obviously a 1.25” eyepiece will not fit straight into a 0.965” focuser - an adapter (see image #10) is  required allowing the telescope to take the larger  eyepieces. See the bottom of this page for alternative eyepieces. Magnification (M) can be calculated by     M= focal length of the telescope            focal length of the eyepiece   For example a telescope of 700mm focal length (FL) and an  eyepiece of 4mm gives you an M = 175  (ie 700/4). This is  written as 175x - meaning 175 times magnification  For a telescope of 70 or 76mm diameter this is much too high.  The image will be faint and probably will not focus sharply.  The good guide to the maximum magnification of any telescope  can be found by dividing the aperture (telescope front lens or  mirror diameter) by 25 then multiply by 30  eg 76mm reflector telescope  76/25 = 3  3 x 30 = 90  If your maximum magnification does not exceed 90 your  telescope will give you enjoyable bright, sharp views. Use the first calculation to see what magnification each of your  eyepieces will give. Your kit may also include a 2x (or 3x) Barlow  lens. This is a special lens which when used with an eyepiece  doubles (2x) or triples (3x) the normal magnification of that  eyepiece. Here is an example of the above:  You have a 76/700 reflecting telescope - this means the mirror is  76mm in diameter with a focal length of 700mm  There are 3 eyepieces with labels reading 20mm, 12.5mm & 4mm  - this means focal lengths of these eyepieces are 20mm 12.5mm  & 4mm  There is also a 2 x Barlow - this will triple the magnification of  any of the eyepieces Eyepiece FL Magnification 20mm  35x  (=700/20)  20mm + 2x Barlow  70x (=35x2)  12.5mm  56x  (=700/20)  12.5mm + 2x Barlow  112x (=35x2)  4mm  175x  (=700/20)  4mm + 2x Barlow  350x (=35x2)  You may think a magnification of 35x will not show you much, but  it depends on the size of the object. The Moon will show a lot of  detail and allow you to decide where you what to look more  closely. If you have not too much light pollution large star  clusters will fill the view. As you will see the 4mm eyepiece on its own far exceeds the 90x  maximum for this size of telescope. Although the use of the  Barlow lens with the 12.5mm eyepiece exceeds this, on bright  objects like the Moon you may find the view is OK. A quick calculation will show that a 20mm eyepiece with a 3x  Barlow lens gives 105x magnification, which means you could not  use it with any of the other eyepieces. Better Eyepieces A quick search of the internet for Eyepieces will soon show you a  confusing number of types and focal lengths, so which one is best for you? First off don’t spend a huge amount on an eyepiece. An  eyepiece costing as much as your telescope will not improve the  view as much as you might hope. It is more sensible to save that  money for a better telescope in the future. Nor do you need a box full of eyepieces - two or at most three covering low, medium and high magnification is all you need. If you are happy with the  20mm eyepiece that came with the telescope why replace it? It is the mid and high magnification eyepieces and probably the  Barlow lens too that usually need replacing.  So what other sizes do you need? You can work it out by  reversing the magnification equation.  Eyepiece FL = Focal length of the telescope           Magnification Eg You want 90x magnification 700/90 = 7.7mm  Eyepieces FL are usually whole numbers so you cannot buy a 7.7  mm eyepiece only a 7mm or 8mm. Rather than buy one that size  you can achieve the same magnification using the Barlow lens  and you get the advantage of the lower magnification using it on  its own too. Therefore, I would suggest a 15mm eyepiece. This  gives 47x on its own and 94x with the Barlow lens.  If the 20mm eyepiece is just adequate, but you would like to  change it then buy a 25mm eyepiece as it will give you lower  magnification and a larger field of view to locate an object. A  25mm eyepiece gives 28x on its own and 56x with the Barlow  lens. This later is the same as the 12.5mm eyepiece that came  with the telescope so you have effectively replaced that too. So with just two new eyepieces and possibly a new Barlow lens  you have a wide range of magnification from low to the maximum  the telescope can easily handle. The industry standard for eyepieces is a barrel diameter of 1.25”  so they can be used in any telescope you buy in the future. How much do eyepieces cost? We have a special offer for you:   25mm Eyepiece  £18  15mm Eyepiece  £18  Basic 2x Barlow Lens  £10  Higher Quality 2x Barlow Lens  £25  Red Dot Finder (2 hole type)  £15  0.965” to 1.25” Eyepiece Adaptor £12 Delivery charges apply 
If you need any help or advice on modifications, eyepieces  or buying a new telescope please contact me.
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